Process and device for the removal of H2 S

ABSTRACT

Process for removing hydrogen sulfide at low concentration in a gas containing water vapor. The gas is cooled to a temperature below the dew point of water, which condenses and separates the water. The dry gas is scrubbed free of hydrogen sulfide by an H 2  S-absorbent solvent forming a purified gas. The purified gas is contacted and rehydrated with heated water so that the water content as water vapor corresponds to a mass flow rate of water substantially equal to the mass flow rate of the water present in the H 2  S-containing gas before cooling. The condensed water circulates in a closed circuit to contact and rehydrate the purified gas, and then is cooled to the appropriate temperature for cooling the H 2  S-containing gas. An apparatus, appropriate for carrying out the process is disclosed.

The invention relates to a process for the removal of H₂ S present atlow concentration in a gas which also contains a large quantity of watervapour. It also relates to a device for making use of this process.

Gases containing H₂ S at low concentration, namely in a concentrationranging from 10 volumes per million to 10% by volume, and alsocontaining a quantity of water vapour ranging from 5 to 60% by volumeare frequently encountered in industry. This is particularly the casewith certain residue gases and especially residue gases of a sulphurplant, which have been subjected to a hydrogenation and hydrolysistreatment to bring all the sulphur compounds which they contain into thesingle H₂ S form, or else synthetic gases which are obtained, forexample, by hydrogenation or cracking of carbonaceous products such ascoal, wood or petroleum residues

Such gases are generally subjected to a treatment for the removal of theH₂ S which they contain either, when synthesis gas is involved, toprevent this compound from perturbing the synthesis reactions in whichthese gases are to take part, or, in the case of residue gases, to bringthese gases within the standards imposed by legislation concerningatmospheric pollution before they are discharged into the atmosphere.

In a known process (FR-A-2,101,724), which is employed for the treatmentof residual gases orieinating from a Claus sulphur plant and whichprimarily comprises stage of hydrogenation and hydrolysis of sulphurcompounds other than H₂ S which are present in the said residue gases tobring these sulphur compounds into the single form of H₂ S, the gasstream resulting from the said hydrogenation and hydrolysis stage, whichhas an H₂ S content of less than 5% by volume and water vapourconcentration of the order of 30% by volume, is cooled to a temperaturebelow the dew point of the water which it contains, to condensevirtually all this water. The water-free gas stream is then scrubbedcountercurrentwise, be operating at a temperature below 50° C., with aregenerable solvent binding H₂ S, for example an aqueous solution of asecondary or tertiary alkanolamine, and the scrubbed gas stream is thensubjected to an incineration to convert into SO₂ the last traces of H₂ Swhich it may still contain, and is then discharged into the atmosphere.

In a process of this kind, the water condensed during the stage ofcooling the hydrogenated gas stream is an acidic water saturated with H₂S, which must undergo a specific purification treatment to be capable ofbeing reemployed in the process or discharged as waste water, atreatment of this kind being complex and by the same token costly,especially because of the large volumes of water to be treated.

To avoid the disadvantages due to the condensation of water, referenceFR-A-2,425,886 proposes to do away with this condensation by permanentlymaintaining the temperature of the gas stream at all points of the plantat a value above the dew point of the water contained in the said gasstream.

When operating in this way, it is necessary for the contact between thehydrogenated gas stream and the regenerable solvent to be brought aboutat higher temperatures, namely between 65° C. and 85° C., and,furthermore, all the water vapour is present in the gas stream when thiscontact takes place. These operating conditions result in the use of amarkedly higher solvent flow rate, namely approximately three timesmore, to obtain the same degree of removal of H₂ S.

In the process of reference FR-A-2,449,470, the stage of condensation ofthe water contained in the gas stream to be treated originating from thehydrogenation is maintained, before the said gas stream is brought intocontact with the regenerable solvent, but the condensed water isinjected into the circuit of the said solvent and the temperature ofthis solvent is adjusted to a suitable value of the order of 70° C. whenit is injected into the zone for bringing into contact with the gasstream containing H₂ S, to bring about an equilibrium in the overallwater balance of the said zone so as to avoid any discharge of liquidwater.

In such a form of application, the flow rates of regenerable solvent tobe employed to obtain the desired degree of removal of H₂ S arecomparable to those which must be employed in the process of referenceFR-A-2,425,886.

The invention proposes a process for the removal of H₂ S present at lowconcentration, namely 10 v.p.m. (volumes per million) to approximately10% by volume, in a gas also containing 5 to 60% by volume of watervapour and possibly other gases such as CO₂. Like the process ofreference FR-A-2,101,724, a process of this kind comprises a separationby condensation of the water contained in the gas to be treated,followed by bringing the gas to be treated, freed from its water, intocontact with a regenerable solvent binding H₂ S, but it does not requireany purification treatment of the condensed water and therefore makes itpossible to get rid of the disadvantages connected with such a treatmentIn addition, the process according to the invention also makes itpossible to avoid the high solvent circulation flow rates which arenecessary in the variants described in references FR-A-2,425,886 and2,449,470, to obtain a comparable H₂ S removal.

The process according to the invention is of the type in which the gasto be treated is cooled to a temperature below the dew point of thewater which it contains so as to separate from the said gas bycondensation, virtually all its water and the cooled gas, rid of itswater, is scrubbed with an absorbent liquid binding H₂ S and regenerableto form an absorbent liquid charged with H₂ S, which is regenerated andreemployed, and a purified gas substantially free from water and from H₂S, and it is characterized in that the said purified gas is brought intocontact with a sufficient quantity of water which contains the waterproduced during the cooling of the gas to be treated and optionallyother waste waters and which is heated to an appropriate temperaturehigher than that of the purified gas, so as to produce a rehydratedpurified gas whose temperature is such that the quantity of water whichit contains corresponds to a mass flow rate of water substantially equalto the mass flow rate of the water present in the gas to be treated,before the latter is cooled, and of the optionally added waste waters.

The process according to the invention makes it possible to treatvarious gases which contain H₂ S at low concentration, namely 10 v.p.m.to approximately 10% by volume and more particularly 50 v.p.m. toapproximately 6% by volume, as well as a quantity of water vapourranging from 5 to 60% by volume and more particularly from 10 to 50% byvolume, it being possible for the said gases also to contain otheracidic compounds such as CO₂. The gases to be treated may consistparticularly of certain natural gases, of synthetic gases produced, forexample, by hydrogenation or cracking of carbonaceous products such ascoal, wood or petroleum residues, or else consisting of residue gases.The said process is applicable more particularly to the treatment ofresidual gases from sulphur plants operating in accordance with theClaus process, after the said residue gases have undergone ahydrogenation and hydrolysis treatment to bring all the sulphurcompounds which they contain into the single form of H₂ S, the saidhydrogenation and hydrolysis treatment being carried out as described,for example in French Patent No. 85/15,905 (publication no. 2,589,141)of 25 Oct. 1985. The temperature of the gases to be treated may range,for example, from approximately 30° C. to approximately 150° C., whileits pressure, which is generally that at which this gas is available,may range from atmospheric pressure to approximately 50 bars.

The water used to rehydrate the purified gas circulates in a closedcircuit, the said circuit being fed with the condensed water producedduring the cooling of the gas to be treated and optionally with wastewater from other sources and giving up water to the purified gas whenthe latter is rehydrated.

As indicated above, the temperature of the water in the said circuit isadjusted upstream of the rehydration zone, that is to say the zone wherethe water is brought into contact with the purified gas to berehydrated, to a value higher than that of the said gas and sufficientfor the rehydrated purified gas, as it leaves the rehydration zone, tocontain a quantity of water corresponding to a mass flow rate of watersubstantially equal to the mass flow rate of the water present in thegas to be treated, before the latter is cooled, and of the optionallyadded waste waters.

Cooling of the gas to be treated below the dew point of the water whichit contains may be carried out in any known manner. It is possible, inparticular, to . perform this cooling by direct or indirect heatexchange with the water of the purified gas rehydration circuit, as itleaves the rehydration zone and after it has been cooled to theappropriate temperature.

The absorbent liquid employed in the scrubbing stage to bind the H₂ Spresent in the cooled gas, which is substantially rid of its water, maybe chosen from the various liquids which absorb H₂ S and which areregenerable, especially by heating. The absorbent liquid may be based onone or more solvents which act physically, such as methanol,polyethylene glycol dimethyl ether, N-methylpyrrolidone, sulpholane,phosphoric esters, or else may consist of a chemically acting solventconsisting of an aqueous solution of one or more compounds bindingacidic gases such as H₂ S in the form of thermally unstable complexes orsalts, such as, for example, an aqueous solution of one or morealkanolamines such as methyldiethanolamine, triethanolamine,diisopropanolamine, monoethanolamine, diethanolamine, diglycolamine andstearically hindered amines. The absorbent liquid may also be chosenfrom mixtures of two abovementioned types of solvent such as, forexample, mixtures of water, diisopropanolamine and sulpholane, mixturesof water, methyldiethanolamine and sulpholane and mixtures of water,methanol and one or more amines such as methyldiethanolamine,monoethanolamine, diethanolamine and diisopropanolamine. When the gas tobe treated contains in addition to H₂ S, other acidic gases andespecially CO₂, the absorbent liquid is preferably chosen from those ofthe absorbent liquid like those mentioned above, which absorb H₂ Sselectively. Most especially suitable as an absorbent liquid which isselective for H₂ S and regenerable by heating is an aqueous solution ofan alkanolamine chosen particularly from methyldiethanolamine,triethanolamine and diisopropanolamine or of an amine chosen fromcertain stearically hindered amines, the alkanolamine or hindered amineconcentration of this solution being, for example, between 1N and 8N andpreferably lying between 3N and 6N.

The scrubbing of the cooled gas using the absorbent liquid is carriedout in a scrubbing zone in which the gas to be scrubbed and theabsorbent liquid preferably circulate countercurrentwise.

The pressure prevailing in the scrubbing zone corresponds substantiallyto that of the gas to be scrubbed injected into the said zone. Thetemperature to be chosen to perform the scrubbing, which depends interalia on the nature of the absorbent liquid employed for binding the H₂S, lies below the dew point of the water in the gas to be treated. Thisscrubbing temperature is generally below 70° C. and, when the absorbentliquid is chosen from aqueous alkanolamine solutions, it isadvantageously between 5° C. and 55° C. The temperature prevailing inthe scrubbing zone corresponds substantially to that of the absorbentliquid introduced into the said zone.

Regeneration of the absorbent liquid charged with H₂ S can be carriedout by decompression, by stripping with an inert gas and/or by heating,depending on the nature of the absorbent liquid, the operatingconditions for applying the chosen regeneration technique being thoserecommended in the art for the technique in question.

A device for making use of the process according to the inventioncomprises a system for cooling the gas to be treated, which is providedwith a delivery conduit for the gas to be treated and which also has anexit for the gases and an exit for the liquids, a scrubbing column whichhas an overhead exit for the gases, an entry for the regeneratedabsorbent liquid in its upper part and, in its lower part an exit forthe charged absorbent liquid and a gas entry, the latter being connectedto the gas exit of the cooling system, and a regeneration column whichhas an overhead exit for an acidic gas stream, an exit for theregenerated absorbent liquid at the bottom, this exit being connected bya conduit to the absorbent liquid entry of the scrubbing column, and, inits upper part, an entry for the charged absorbent liquid, the saidentry being connected by a conduit to the corresponding exit of thescrubbing column, and it is characterized in that it also comprises, onthe one hand, a rehydration column which has a gas exit overhead, aliquid entry in its upper part and in its lower part a liquid exit and agas entry, this entry being in communication with the gas exit of thescrubbing column, and, on the other hand, a rehydration water circuithaving one end connected to the liquid exit of the rehydration columnand another end connected to the liquid entry of this column, the saidcircuit being combined with the cooling system so that, on the one hand,the water condensed by the cooling of the gas to be treated flows viathe liquid exit of the cooling system. into the rehydration circuit andthat, on the other hand, the rehydration water of this circuit may beheated by heat exchange with the gas to be treated, the rehydrationwater circuit being also provided with means for ensuring thecirculation and the cooling of the water which it contains, these meansbeing arranged between the exit of the rehydration column and thecooling system, and optionally means for introducing waste waters

According to an embodiment, the system for cooling the gas to be treatedcomprises an indirect heat exchanger and a condensation flask which hasan entry and is provided with a gas exit and a liquid exit, the saidexits constituting the gas and the liquid exits of the cooling systemrespectively, one of the circuits of the indirect heat exchanger formingpart of the rehydration water circuit while the other circuit of thesaid exchanger connects the delivery conduit for the gas to be treatedto the entry of the condensation flask.

According to another embodiment, the cooling system consists of acooling column which is provided with a gas exit and a liquid entry inits upper part and with the delivery conduit for the gas to be treatedand a liquid exit in its lower part, the said gas and liquid exitsforming the gas and the liquid exits, of the cooling systemrespectively, the said cooling column being mounted in the rehydrationwater circuit via its liquid entry and exit. In an alternative form ofthis embodiment, the cooling column is equipped with an indirect heatexchange apparatus comprising an entry and an exit and whose entry formsthe liquid entry of the said cooling column, the said apparatus beingmounted via its entry and exit in the rehydration water circuit.

The cooling and rehydration columns may also constitute two superposedand independent sections of the same single column, namely a coolingsection supporting a rehydration section, it being possible for thecooling section to have either of the structures defined above for thecooling column.

In another embodiment, the cooling, scrubbing and rehydration columnsform three superposed sections of the same single column, namely a lowercooling section, an intermediate scrubbing section and an upperrehydration section, so that the gas exit of one section coincides withthe gas entry of the section situated immediately above, the coolingsection having either of the structures defined above for the coolingcolumn and being connected to the rehydration water circuit as indicatedfor the said column.

The columns or sections of the device according to the invention inwhich a direct gas-liquid contact is produced are generally providedwith trays or with a packing of any known type, which are intended toimprove the gas/liquid contact. The number of trays or the height of thepacking of the column or section in question is chosen so that the saidcolumn or section may have the desired operating performance.

Other characteristics and advantages will appear on reading thedescription of the forms of application of the process according to theinvention, which is given below with reference to the figures of theattached drawing, in which:

FIGS. 1 and 2 show two alternative forms of a first device according tothe invention comprising the cooling, scrubbing and rehydration sectionsin the same single column;

FIGS. 3 and 4 show two alternative forms of a second device according tothe invention comprising the cooling and rehydration sections in thesame single column and the scrubbing section in a separate column; and

FIG. 5 shows a third device according to the invention, comprising threeseparate units for cooling, scrubbing and rehydration.

The device shown in FIG. 1 comprises a column 1 divided into threesections, namely a lower cooling section 2, an intermediate scrubbingsection 3 and an upper rehydration section 4, each of the said sectionsbeing provided with trays for gas-liquid contact. Cooling section 2communicates with the scrubbing section 3 via a passage 5 for the gases,the said passage forming the overhead gas exit of the cooling sectionand at the same time the entry for the gases at the bottom of thescrubbing section, and the scrubbing section 3 communicates with therehydration section 4 via a passage 9 for the gases, the said passageforming at the same time the overhead gas exit of the scrubbing zone andthe gas entry situated in the lower part of the rehydration section.Cooling section 2 comprises, moreover, an entry 6 for the liquids in itsupper part and an exit 7 for the liquids at its bottom and, in addition,a delivery conduit 8 for the gas to be treated opens into the lower partof the said section. Scrubbing section 3 also comprises an entry 10 forthe regenerated absorbent liquid in its upper part and an exit 11 forthe charged absorbent liquid in its lower part. The rehydration section4 has an overhead gas exit 12 extended by a gas removal conduit 13, aliquid entry 14 in its upper part and a liquid exit 15 in its lowerpart. Cooling section 2 is mounted via its liquid entry 6 and exit 7 ina rehydration water circuit which comprises, on the one hand, a conduit16 provided with an indirect heat exchanger 17 and connecting the liquidexit 15 of the rehydration section to the liquid entry 6 of the coolingsection and, on the other hand, a conduit 18 connecting the liquid exit7 of the cooling section to the liquid entry 14 of the rehydrationsection. The conduit 18 is provided with a pump 19 whose delivery isdirected towards the entry 14 of the rehydration section. A regenerationcolumn 20, combined with the scrubbing section 3, comprises an exit 21for an acidic gas overhead, an entry 22 for the absorbent to beregenerated in its upper part and an exit 23 for the regeneratedabsorbent at the bottom. The entry 22 of the regeneration column isconnected to the exit 11 of the scrubbing section via a conduit 24, inwhich is mounted the cold circuit of an indirect heat exchanger 25. Theexit 23 of the regeneration column is connected to the entry 10 of thescrubbing section via a conduit 26 on which the hot circuit of the heatexchanger 25 is mounted. In addition, the regeneration column isconnected in its lower part via entry 27 and exit 28 pipes to a reboiler29 heated by indirect heat exchange by means of steam circulating in apipe 30. In addition, the exit 21 of the regeneration column isconnected via a pipe 31 to a condenser 32, which is provided, in itsupper part, with a conduit 33 for removal of acidic gas and whose lowerpart is connected to the upper part of the regeneration column by aconduit 34 ensuring the return of the condensates

The device of FIG. 2 shows an alternative form of the device showndiagrammatically in FIG. 1. In this alternative form, the elements whichare identical with the elements of the device of FIG. 1 are indicated bythe same reference signs. In relation to the device of FIG. 1, thecooling section 2 of the device of FIG. is no longer fitted with traysfor gas/liquid contact, but is equipped internally with an apparatus 35for cooling by indirect heat exchange, the said apparatus 35 having aliquid entry 36, forming the liquid entry of the cooling section 2 andconnected to the liquid exit 15 of the rehydration section 4 by theconduit 16, equipped with an air cooler by way of heat exchanger 17, anda liquid exit 37 connected to the conduit 18 by a conduit 38 in whichthere is mounted a pump 39 whose delivery is directed towards theconduit 18. In addition, as in the case of the device of FIG. 1, theliquid entries 10 and exit 11 of the scrubbing section 3 are connected,by the conduits 26 and 24 respectively, to the liquid exit 23 and entry24 of a regeneration column 20, not shown and identical with that showndiagrammatically in FIG. 1.

The device shown in FIG. 3, and respectively in FIG. 4, differs from thedevice shown in FIG. 1, and respectively in FIG. 2, only in that thescrubbing section 3 forms an independent scrubbing column and that thecooling section 2 and the rehydration section 4 are two superposedsections of the same single column 41, the gas exit 5a of the coolingsection 2 being connected by a conduit 42 to the gas entry 5b of thescrubbing column 3 and the gas exit 9b of the said column 3 beingconnected by a conduit 43 to the gas entry 9c of the rehyration section4. The other components of the device of FIG. 3, and respectively of thedevice of FIG. 4, are identical with the components bearing the samereferences of the device of FIG. 1, and respectively of the device ofFIG. 2. In particular, the liquid entry 10 and exit 11 of the scrubbingcolumn 3 are connected, by conduits 26 and 24 respectively, to theliquid exit 23 and entry 22 of a regeneration column 20, not shown andidentical with that shown diagrammatically in FIG. 1.

The operation of the device shown in any one of FIGS. 1 to 4 may beoutlined as follows.

The gas to be treated, which contains the H₂ S to be extracted and alarge quantity of water vapour, is delivered by the conduit 8 to thecooling section 2 of the column 1 (FIGS. 1 and 2) or of the column 41(FIGS. 3 and 4) and in the said section meets, either directly (FIGS. 1and 3) or indirectly (FIGS. 2 and 4) a stream of water delivered by theconduit 16 originating from the rehydration section 4 and cooled to asuitable temperature in the heat exchanger 17 mounted in the conduit 16and operating as a cooler. The temperature of the water stream enteringthe cooling section 2 is adjusted, by virtue of the cooler 17, so as tocondense virtually all the water contained in the gas delivered by theconduit 8 and so as to take the said gas to a temperature correspondingsubstantially to that of the scrubbing, which results in obtaining, atthe bottom of section 2, a water stream heated by the sensible heatgiven to it by the gas during its cooling, to which is added the latentheat of the condensed water.

The gas, cooled and substantially rid of its water, enters, via thepassage 5 (FIGS. 1 and 2) or via the conduit 42 and the passage 5b(FIGS. 3 and 4), the scrubbing section or column 3, in which the saidgas is subjected to a countercurrent scrubbing using the absorbentliquid injected into the said section or column 3 via the entry 10. Theabsorbent liquid charged with H₂ S is drawn off from the section orcolumn 3 via the exit 11 and is then directed by the conduit 24, whilebeing heated in the exchanger 25, into the regeneration column 20, to beregenerated by reboiling therein, the regenerated absorbent liquid beingrecycled to the entry 10 of the section or column 3 by the conduit 26.An acidic gas stream containing H₂ S, which may be sent, for example, toa Claus sulphur plant, is removed by the conduit 33 of the condenser 32of the regeneration column.

The gas purified by scrubbing in the section or column 3 enters, via thegas passage 9 (FIGS. 1 and 3) or via the conduit 43 and the passage 9c,the rehydration section 4 of column 1 (FIGS. 1 and 3) or of column 41(FIGS. 3 and 4), in which it meets counter-currentwise an excess ofwater injected through the entry 14 for the liquids from the rehydrationsection, the said water being delivered by the conduit 18 at atemperature higher than that of the gas entering the said section 4 andsufficiently high to remove, through the exit 12 of the rehydrationsection 4 and the conduit 13, a rehydrated purified gas which contains aquantity of water corresponding to a mass flow rate of watersubstantially equal to the mass flow rate of the water present in thegas to be treated before the latter is cooled. The water not employedfor the rehydration of the purified gas leaves the section 4 by the exit15, the said water being returned to the cooling section 2 by theconduit 16 and at a temperature which is adjusted by the cooler 17.

A continuous and controlled water circulation is therefore establishedin a closed circuit between the section 2 for cooling the gas to betreated and the section 4 for rehydrating the purified gas, so as totransfer continuously to the purified gas a quantity of water which issubstantially equal to that formed by condensation of the water vapourcontained in the gas to be treated, while short-circuiting the scrubbingsection or column 3.

The adjustment of the temperature of the water injected by the conduit18 into the rehydration section 4 can be carried out, as is the casewith the device of FIG. 1 or of FIG. 3, by adjusting the temperature ofthe water injected into the cooling section 2 via the conduit 16, bycontrolling the cooler 17 so as to obtain at the exit 7 of the coolingsection 2 a stream of water which has the desired elevated temperaturefor its injection into the rehydration section 4 via the conduit 18.

The temperature of the water injected via the conduit 18 into therehydration section 4 can also be adjusted to the appropriate value, asis possible with the device of FIG. 2 or of FIG. 4, by controlling therelative flow rates of the water streams at different temperaturesoriginating, on the one hand, from the cooling system 35 and, on theother hand, from the bottom 7 of the cooling section 2 (condensed water)

The device shown in FIG. 5 comprises a condensation flask 2a, ascrubbing column 3 and a rehydration column 4, the said columns beingseparate from each other and each being provided with trays forgas/liquid contact. The condensation flask 2a has an overhead exit 5afor the gases and at the bottom an exit 7 for the liquids and, emerginginto its lower part, a delivery conduit 8 for gas to be treated, inwhich the hot circuit of an indirect heat exchanger 46 is mounted. Thescrubbing column 3 has a gas exit 9b overhead, an entry 10 for theregenerated absorbent liquid in its upper part, a gas entry 5b in itslower part and an exit 11 for the absorbent liquid charged with H₂ S atthe bottom The rehydration column 4 has an overhead gas exit 12,extended by a gas removal conduit 13, a liquid entry 14 in its upperpart, a gas entry 9c in its lower part and a liquid exit 15 at thebottom. The gas exit 5a of the flask 2a is connected to the gas entry 5bof the column 3 by a conduit 42 and the gas exit 9b of the said column 3is connected to the gas entry 9c of the column 4 by a conduit 43. Aconduit 44, forming a water circuit, is connected at one of its ends tothe liquid exit 15 of the rehydration column 4 and at its other end tothe liquid entry 14 of the said column 4. The cold circuit of the heatexchanger 46 is mounted in the conduit 44, the said conduit 44 beingalso equipped, on the one hand, with a pump 45, which is arrangedupstream of the exchanger 46 and whose delivery is directed towards thesaid exchanger and consequently towards the entry 14 of the column 4and, on the other hand, with an indirect heat exchanger 40 arrangedupstream of the pump 45. In addition, the exit 7 at the bottom of theflask 2a is connected to the conduit 44, downstream of the heatexchanger 46, by a conduit 47 in which there is mounted a pump 48 whosedelivery is directed towards the conduit 44. Moreover, as in the casefor the devices of FIGS. 1 to 4, the liquid entry 10 and exit 11 of thescrubbing column 3 are connected, by the conduits 26 and 24respectively, to the liquid exit 23 and entry 22 of a regenerationcolumn 20, not shown and identical with that of FIG. 1.

The operation of the device shown in FIG. 5 is comparable with that ofthe devices shown diagrammatically in FIGS. 1 to 4.

The gas to be treated, which contains the H₂ S to be removed and a largequantity of water vapour, is delivered by the conduit 8 and passesthrough the indirect heat exchanger 46 before entering the condensationflask 2a. A stream of water travels continuously in the rehydrationcolumn 4 and then in the conduit 44, from the liquid exit 15 of the saidcolumn 4, to return to the column 4 via the entry 14 of the latter,after having passed through the heat exchanger 46. In this heatexchanger 46 the gas to be treated is cooled to the appropriatetemperature below the dew point of the water which it contains bytransferring its heat to the colder water stream carried by the conduit14 and the said water stream is raised to a temperature above the saiddew point. In the flask 2a, the cooled gas separates from the waterwhich has condensed while the gas to be treated passed through the heatexchanger 46.

The cooled gas, substantially rid of its water leaves the flask 2athrough the gas exit 5a and, after passing through the conduit 42,enters, through the gas entry 5b, the scrubbing column 3, in which thesaid gas is subjected to countercurrent scrubbing by the absorbentliquid injected into the said column 3 through the entry 10. Theabsorbent liquid charged with H₂ S is drawn off from the column 3 viathe exit 11 and is then directed by the conduit 24, while being heatedin the exchanger 25, into the regeneration column 20, to be regeneratedtherein by reboiling, the regenerated absorbent liquid being recycled tothe entry 10 of the column 3 by the conduit 26. Stream of acidic gascontaining H₂ S is removed via the conduit 33 of the condenser 32 of theregeneration column, the said stream being, for example, conveyer to aClaus sulphur plant.

The gas rid of H₂ S by scrubbing in the column 3 leaves the said columnthrough the gas exit 9b and, after passing through the conduit 43,enters, through the gas entry 9c, the rehydration column 4, in which itmeets countercurrentwise an excess of water injected through the liquidentry 14 of the rehydration column 4. This water is delivered by theconduit 44 at a temperature which is higher than that of the gasentering the said column 4 through the gas entry 9c, the saidtemperature being sufficiently elevated for removing, through the exit12 of the column 4 and the conduit 13, a rehydrated purified gascontaining a quantity of water corresponding to a mass flow rate ofwater which is substantially equal to the mass flow rate of the waterpresent in the gas to be treated, before the latter is cooled. The waternot employed for rehydrating the purified gas leaves the column 4through the exit 15, the said water being returned to the column 4 bythe conduit 44, as shown previously, after having been first of allcooled in the indirect heat exchanger 40 and then heated in the heatexchanger 46 and after having had added to it, via the conduit 47, therequired quantity of condensed water taken from the flask 2a through theliquid exit 7.

The adjustment of the temperature of the water injected via the conduit44 into the rehydration column 4 is carried out by controlling the flowrates of water in conduits 44 and 47 by regulation of the pumps 45 and48 which are mounted in the said conduits

This application also results in transferring continuously to thepurified gas a quantity of water which is substantially equal to thatformed by condensation of the water vapour contained in the gas to betreated, this being done without passing this water through thescrubbing column 3.

To supplement the above description, two concrete examples ofapplication of the process according to the invention are given below byway of illustration and without any limitation being implied.

EXAMPLE 1

A residual gas containing 0.35% of H₂ S, 1.65% of CO, 25% of CO₂, 29% ofwater vapour and 44% of nitrogen, by volume, was treated by operating ina device similar to that shown in FIG. 1, the said gas resulting fromthe hydrogenation and hydrolysis of a residual gas from a Claus sulphurplant.

In the device employed, each of the cooling 2, scrubbing 3 andrehydration 4 sections of the column 1 contained 10 trays for gas/liquidcontact and the regeneration column 20 contained 20 of these.

The absorbent liquid employed in the scrubbing section 3 consisted of a4N aqueous solution of methyldiethanolamine (abbreviated to MDEA), aselective absorbent for H₂ S.

The gas to be treated was delivered to the cooling section 2 at a flowrate of 160,000 Nm³ /h, a temperature of 110° C. and an absolutepressure of approximately 1.3 bars and met countercurrentwise in thesaid cooling section 2 a stream of water injected through the liquidentry 6 of this section at a flow rate of 450 m³ /h and a temperature of15° C. On contact with the stream of water injected through the entry 6of the cooling section 2, virtually all of the water contained in thegas to be treated condensed and mixed with the said stream producine, atthe bottom of the said section 2, water at a temperature of 93° C. and,at the head of this section, a cooled gas at a temperature of 20° C. andsubstantially free from water.

The cooled gas entered, through the passage 5, the scrubbing section 3at a temperature of 20° C. and met countercurrentwise, in this section3, the regenerated absorbent liquid injected through the entry 10 at aflow rate of 800 m³ /h and a temperature of 20° C., the said absorbentliquid being delivered from the regeneration column 20 by the conduit26. The absorbent liquid charged with H₂ S left the scrubbing section 3by the exit 11, at a flow rate of 800 m³ /h, and was lead to theregeneration column 20, in which, for the purpose of regeneration, itwas raised at the bottom of this column 20 to temperature ofapproximately 125° C. by means of steam circulating in the pipework 30of the reboiler. The regenerated absorbent liquid was returned to theentry 10 of the scrubbing section 3 by the conduit 26.

The purified gas entered, through the passage 9, the rehydration section4 at a temperature of 20° C. and met countercurrentwise a stream ofwater injected into this section through the entry 14 at a temperatureof 93° C. and a flow rate of 500 m³ /h, the said stream of water beingdelivered by the conduit 18, by the action of the pump 19, from thebottom 7 of the cooling section 2. A rehydrated purified gas containingless than 100 v.p.m. (volumes per million) of H₂ S and exhibiting awater dew point of 70° C. was removed at a flow rate of 150,000 Nm³ /hby the conduit 13 extending the overhead gas exit 12 of the rehydrationsection 4, the said temperature being very close to the dew point of thewater in the gas to be treated delivered by the conduit 8. The water notemployed in the rehydration section 4 left the said section through theexit 15 at a flow rate of 450 m³ /h and a temperature of 23 ° C. Thiswater was lead, via the conduit 16, to the entry 6 of the coolingsection 2 after having been cooled to 15° C. by passing through thecooler mounted in the conduit 16.

The rehydrated purified gas leaving through the discharge conduit 13removed continuously a quantity of water vapour corresponding to a massflow rate of water substantially equal to the mass flow rate of thewater condensed in the section 2 by cooling the gas to be treated. Therehydrated purified gas then underwent an incineration to convert thelast traces of H₂ S which it contained into SO₂, and was then dischargedinto the atmosphere.

EXAMPLE 2

A residual gas having the same composition as the gas treated in Example1 was treated by operating in a device similar to that shown in FIG. 2,this residual gas also originating from the hydrogenation and hydrolysisof a residual gas of a Claus sulphur plant.

In the device employed, the cooling section 2 was equipped with anindirect heat exchange apparatus 35, the washing 3 and rehydration 4sections contained 10 and 6 trays for gas/liquid contact respectively,and the regeneration column 20 contained 20 of these.

The absorbent liquid employed was identical with that employed inExample 1.

The gas to be treated was delivered to the cooling section 2 at a flowrate of 160,000 Nm³ /h, a temperature of 120° C. and an absolutepressure of 1.3 bars and was cooled, in the said section 2, to atemperature of 50° C. by indirect heat exchange with the stream of waterinjected into the heat exchange apparatus 35, through the entry 36 ofthe latter, at a flow rate of 800 m³ /h and a temperature of 45° C. Onindirect contact with the stream of water injected through the entry 36of the exchanger apparatus 35, virtually all the water contained in thegas to be treated condensed, producine, at the bottom of the section 2,a stream of condensed water at a temperature of 90° C. and, at the headof this section, a cooled gas at a temperature of 50° C. andsubstantially free from water, a stream of hot water at a temperature of90° C. being also obtained at the exit 37 of the exchanger apparatus 35.

The cooled gas entered, through the passage 5, the scrubbing section 3at a temperature of 50° C. and met countercurrentwise, in this section,the regenerated absorbent liquid injected through the entry 10 at a flowrate of 1,300 m³ /h and a temperature of 45° C., the said absorbentliquid being delivered from the regeneration column 20 by the conduit26. The absorbent liquid charged with H₂ S left the scrubbing section 3through the exit 11, at a flow rate of 1,300 m³ /h, and was lead to theregeneration column 20 in which, for the purpose of regeneration, it washeated at the bottom of the said column to a temperature ofapproximately 125° C. by means of steam circulating in the pipework 30of the reboiler 29. The regenerated absorbent liquid who returned to theentry 10 of the scrubbing section 3 by the conduit 26.

The purified gas entered, through the passage 9, the rehydration section4 at a temperature of 45° C. and met countercurrentwise a stream ofwater injected into this section through the entry 14 at a temperatureof 90° C. and a flow rate of 840 m³ /h, the said stream being formed bythe merging, on the one hand, of the stream of water flowing at atemperature of 90° C. through the exit 36 of the exchanger apparatus 35and pumped, by the pump 39, in the conduit 38 at a flow rate of 800 m³/h and, on the other hand, of a stream of condensed water drawn off, ata temperature of 90° C., through the exit 7 of the section 2 and pumped,by the pump 19, in the conduit 18, upstream of the conduit 38, at a flowrate of 40 m³ /h.

A rehydrated purified gas containing less than 200 v.p.m. of H₂ S andexhibiting a water dew point of 70° C. was removed at a flow rate of150,000 Nm³ /h via the conduit 13 extending the overhead gas exit 12 ofthe rehydration section 4, the said temperature being very close to thewater dew point of the gas to be treated delivered by the conduit 8. Thewater not employed in the rehydration section 4 left the said sectionthrough the exit 15 at a flow rate of 800 m³ /h and a temperature of 60°C. This water was lead by the conduit 16 to the entry 36 of theexchanger apparatus 35 after having been cooled to 45° C. by passingthrough the air cooler 17 mounted in the conduit 16.

The rehydrated purified gas leaving by the conduit 13 removedcontinuously a quantity of water vapour corresponding to a mass flowrate of water substantially equal to the mass flow rate of the watercondensed in the section 2 by cooling the gas to be treated. Therehydrated purified gas then underwent an incineration, and was thendischarged into the atmosphere.

We claim:
 1. A process for the removal of H₂ S present at lowconcentration in a gas also containing 5 to 60% by volume of watervapor, which comprises the steps of:a) cooling the H₂ S-containing gasto a temperature below the dew point of the water, thereby condensingthe water vapor, separating substantially all of the water and forming acooled H₂ S-containing gas, substantially free of the water, b)scrubbing the cooled H₂ S-containing gas, substantially free of thewater, with a H₂ S-absorbent liquid forming (i) an absorbent liquidcharged with H₂ S and (ii) a purified gas, substantially free of waterand H₂ S, and c) contacting the (ii) purified gas with heated water,said water produced from step a), the temperature of the heated waterbeing such that the purified gas is rehydrated, so that the watercontent as water vapor corresponds to a mass flow rate of watersubstantially equal to the mass flow rate of the waste present in the H₂S-containing gas before cooling said gas, and wherein said waterproduced during step a) circulates in a closed circuit and is cooled toa temperature sufficient for cooling the H₂ S-containing gas andcondensing the water vapor in step a) by direct or indirect heatexchange, after contacting and rehydrating the (ii) purified gas.
 2. Theprocess of claim 1, wherein the heated water contacting and rehydratingthe purified gas in step c) comprises other waste waters, so that thewater content as water vapor corresponds to a mass flow rate of watersubstantially equal to the mass flow rate of the water present in the H₂S-containing gas before cooling said gas and of the added waste waters.3. The process of claim 1, wherein the rehydrated purified gas has awater dew point substantially equal to the water dew point of the H₂S-containing gas before cooling of said gas in step a).
 4. The processof claim 1, wherein the H₂ S-containing gas has a H₂ S content rangingfrom 10 v.p.m. to 10% by volume.
 5. The process of claim 4, wherein theH₂ S-containing gas is a residual gas produced by hydrogenation andhydrolysis of the residual gas from a sulphur plant, wherein all thesulphur compounds are formed into H₂ S.
 6. The process of claim 1,wherein the H₂ S-containing gas also contains CO₂.
 7. The process ofclaim 1, wherein the scrubbing of the cooled H₂ S-containing gas withthe H₂ S-absorbent liquid is performed at a temperature below 70° C. 8.The process of claim 1, wherein the scrubbing of the cooled H₂S-containing gas with the H₂ S-absorbent liquid is performed at atemperature between 5° C. and 55° C.
 9. The process of claim 1, whereinthe absorbent liquid charged with H₂ S, is regenerated and reemployedfor scrubbing of the cooled H₂ S-containing gas in step b).
 10. Theprocess of claim 9, wherein the H₂ S-absorbent liquid selectivelyabsorbs H₂ S.
 11. The process of claim 10, wherein the H₂ S-absorbentliquid selectively absorbing H₂ S, is an aqueous solution of an amineselected from the group consisting of alkanolamines andsterically-hindered amines.
 12. The process of claim 11, wherein thealkanolamines are selected from the group consisting ofmethyldiethanolamine, triethanolamine and diisopropanolamine.
 13. Theprocess of claim 11, wherein the amine concentration of said aqueoussolution is between 1N and 8N.
 14. The process of claim 11, wherein theamine concentration of said aqueous solution is between 3N and 6N. 15.The process of claim 1, wherein substantially all of said condensedwater from step a) contacts and rehydrates the purified gas in step c).16. An apparatus for removing H₂ S present at low concentration in a gasalso containing 5 to 60% by volume of water vapor, which comprises:a)means for cooling the H₂ S-containing gas and for condensing the watervapor by cooling to a temperature below the dew point of the water,wherein substantially all of the water is condensed and separated,thereby forming a cooled H₂ S-containing gas, substantially free of thewater, b) means for scrubbing the cooled H₂ S-containing gas with a H₂S-absorbent liquid so that (i) an absorbent liquid charged with H₂ S isformed, c) means for contacting the purified gas with heated condensedwater from a), said water heated by heating means, wherein the heatedcondensed water is at a temperature for rehydrating the purified gas, sothat the water content as water vapor corresponds to a mass flow rate ofwater substantially equal to the mass flow rate of the water present inthe H₂ S-containing gas, before cooling said gas and d) circulatingmeans for connecting the contacting means and the cooling means, so thatthe water condensed and separated by the cooling means is delivered tothe contacting means for rehydrating the purified gas, then recirculatedto the cooling means of step a).
 17. The apparatus of claim 16, whereinthe cooling means and the contacting means form two superposed anindependent sections of a single column
 18. The apparatus of claim 16,wherein the cooling means, scrubbing means and contacting means formthree supposed sections of a single column, forming a lower coolingcolumn, an intermediate scrubbing column and an upper rehydrationcolumn, so that the gas exit of the cooling column and the gas entry ofthe scrubbing column form a passage for the gases and the gas exit ofthe scrubbing column and the gas entry of the rehydration column form apassage for the gases.
 19. The apparatus of claim 16, wherein the meansfor cooling the H₂ S-containing gas and for condensing the water vaporby cooling to a temperature below the dew point of water, comprises acooling column having, in its upper part, an exit for the cooled H₂S-containing gas, substantially free of the water, and an entry forliquids in its upper part, and in its lower part, a conduit fordelivering the H₂ S-containing gas and an exit for separating thecondensed water.
 20. The apparatus of claim 19, wherein the coolingcolumn is mounted in a rehydration water circuit by the entry for theliquids in its upper part, and the exit for separating the condensedwater in its lower part.
 21. The apparatus of claim 20, wherein thecooling column is equipped with an indirect heat exchanger apparatusmounted between the entry and the exit of the rehydration water circuit.22. The apparatus of claim 16, wherein the means for scrubbing thecooled H₂ S-containing gas with the H₂ S-absorbent liquid comprises ascrubbing column having an entry for a regenerated H₂ S-absorbent liquidin its upper part and an exit for the absorbent liquid charged with H₂ Sin its lower part, and an entry for the cooled H₂ S-containing gas inits lower part and an exit for the purified gas in its upper part. 23.The apparatus of claim 16, wherein the means for contacting the purifiedgas with the heated water, comprises a rehydration column mounted in arehydration water circuit between said contacting zone of c) and the 24.The apparatus of claim 16, which further comprises means forregenerating and reemploying the absorbent liquid charged with H₂ S, sothat the H₂ S-absorbent liquid is regenerated and reemployed for thescrubbing of the cooled H₂ S-containing gas.
 25. The apparatus of claims24, wherein the means for regenerating and reemploying the absorbentliquid charged with H₂ S, so that the H₂ S-absorbent liquid isregenerated and reemployed for the scrubbing of the cooled H₂S-containing gas, comprises a regeneration column, having an overheadexit for an acidic gas, an entry for the absorbent liquid charged withH₂ S in its upper part and an exit for the regenerated H₂ S-absorbentliquid at its bottom.
 26. The apparatus of claim 25, further comprisingmeans for connecting the regeneration column in its lower part to areboiler heated by indirect heat exchange by circulating steam.
 27. Theapparatus of claim 26, further comprising means for connecting theoverhead exit of the regeneration column to a condenser, so that aconduit in the upper part of the condenser removes acidic gas and aconduit in the lower part of the condenser circulates condensates to theupper part of the regeneration column.
 28. The apparatus of claim 25,wherein the means for scrubbing the cooled H₂ S-containing gas with theH₂ S-absorbent liquid, comprises a scrubbing column having an entry fora regenerated H₂ S-absorbent liquid in its upper part and an exit forthe absorbent liquid charged with H₂ S in its lower part, and an entryfor the cooled H₂ S-containing gas in its lower part and an exit for thepurified gas in its upper part.
 29. The apparatus of claim 28, whereinthe regeneration column and the scrubbing column are connected, so thatthe entry of the regeneration column is connected to the exit of thescrubbing column by a conduit, on which is mounted a cold circuit of anindirect heat exchanger, and the exit in the lower part of theregeneration column is connected to the entry of the scrubbing column bya conduit on which a hot circuit of the indirect heat exchanger ismounted.
 30. The apparatus of claim 16, wherein the means for contactingthe purified gas are additionally waste waters heated to a temperaturefor rehydrating the purified gas, so that the heated water content aswater vapor corresponds to a mass flow rate of water substantially equalto the mass flow rate of the water present in the H₂ S containing gasbefore cooling said gas and of the added waste waters from othersources.
 31. The apparatus of claim 16, wherein substantially all thewater condensed and separated by the cooling means is delivered to thecontacting means for rehydrating the purified gas, then recirculated tothe cooling means of step a).
 32. An apparatus for removing H₂ S presentat low concentration in a gas also containing 5 to 60% by volume ofwater vapor, which comprises:a) means for cooling the H₂ S-containinggas and for condensing the water vapor by cooling to a temperature belowthe dew point of the water, wherein substantially all of the water iscondensed and separated, thereby forming a cooled H₂ S-containing gas,substantially free of the water, said cooling means provided with adelivery conduit for the H₂ S-containing gas in its lower part andhaving an exit for the gas in its upper part and an exit for thecondensed water at the bottom, b) means for scrubbing the cooled H₂S-containing gas with a H₂ S-absorbent liquid so that (i) an absorbentliquid charged with H₂ S is formed and (ii) a purified gas,substantially free of the water and H₂ S is formed, which has anoverhead exit for (ii) the purified gas, an entry for the H₂ S-absorbentliquid in it upper part and, in its lower part, an exit for (i) theabsorbent liquid charged with H₂ S and a gas entry in its lower part forthe cooled gas, said gas entry being connected to the gas exit of thecooling means, c) means for contacting the purified gas with heatedcondensed water from step a), said water heated by heating means,wherein the heated condensed water is at a temperature for rehydratingthe purified gas, and so that the water content as water vaporcorresponds to a mass flow rate of water substantially equal to the massflow rate of the water present in the H₂ S-containing gas, beforecooling said gas, which has an overhead exit for the rehydrated purifiedgas, an entry in it upper part for the condensed water from the coolingmeans and, in its lower part, an exit for the rehydration water forcooling the H₂ S-containing gas, and a gas entry in its lower part beingin communication with the gas exit of the scrubbing means, d) means forregenerating and reemploying (i) the absorbent liquid charged with H₂ S,so that the H₂ S-absorbent liquid is regenerated and reemployed for thescrubbing of the cooled H₂ S-containing gas, having an overhead exit foran acidic gas, an entry for (i) the absorbent liquid charged with H₂ Sin its upper part and an exit for the regenerated H₂ S-absorbent liquidat its bottom, and means for connecting the overhead exit of theregenerating means to a condenser, so that a conduit in the upper partof the condenser removes acidic gas and a conduit in the lower part ofthe condenser circulates the condensates to the upper part of theregenerating means, and means for connecting the regenerating means andthe scrubbing means, so that the entry in the upper part of theregenerating means is connected to the exit in the lower part of thescrubbing means by a conduit on which is mounted a cold circuit of anindirect heat exchanger, and the exit in the lower part of theregenerating means is connected to the entry in the upper part of thescrubbing means by a conduit on which a hot circuit of the indirect heatexchanger is mounted and e) circulating means for connecting thecontacting means and the cooling means, so that the water condensed andseparated by the cooling means is delivered to the contacting means forrehydrating the purified gas, said water is then recirculated to thecooling means.